Hi arpan145,
Thanks for using Apple Support Communities! I see you're having an issue with your iPhone camera, and I'd like to help out.
Have you tried cleaning your iPhone with the steps in this article? Cleaning your iPhone
If the dust is behind the lens, a technician would need to take a look at it. There are no user serviceable parts on an iPhone.
You can use this link to find an Apple Store or Apple Authorized Service Provider nearby, and make a reservation.
Find Locations
Prepare our iPhone for service: Get your iPhone, iPad, or iPod touch ready for service
Take care!